A typical ink applicator for a printing roller has at least one upstream or working doctor blade and at least one downstream or closing doctor blade having flexible edges, which may be removable, that can bear radially on the normally cylindrical outer surface of the printing roller. The blades form a normally upwardly open compartment that is filled with the treatment liquid, e.g. ink, that can flow onto the roller surface via the opening formed between the blade edges. The downstream (closing) blade scrapes off excess ink so that what is left in the depressions on the roller surface can be transferred to another roller or directly to the medium being printed, which is pressed against the printing roller at a location offset from the applicator. Alternately in the anilox system the applicator applies the ink to a transfer roller that in turn applies it to the actual printing cylinder, in which case the term “printing” roller refers to this transfer roller.
The closing or upstream blade lying opposite the working or downstream blade serves essentially to seal the opening of an ink chamber with respect to the ink transfer roller when the ink transfer roller is at rest, i.e. when the ink located in the ink chamber is completely distributed in the ink chamber.
During operation when the printing or transfer roller is rotating, this rotation can bring foreign bodies such as dust, grains of sand, chips or other disturbing contaminants entrained by the printing ink remaining on the ink transfer roller after an ink transfer to collect in the area between the upstream blade and the ink transfer roller. Since with known printers the upstream blade, like the downstream blade, bears virtually all the time against the surface of the ink transfer roller with a predetermined force and the upstream blade seen in the direction of rotation of the ink transfer roller as a rule forms an acute angle with the surface of the ink transfer roller, these particles are impossible to remove bodies from under the upstream blade. The result is that they collect there and can scratch or score the roller. Since the downstream blade is oppositely angled, particles do not get wedged under it; instead they simply pass up and remain in the body of ink held in the applicator compartment, where they do no harm.
Moreover, the rotation of the ink transfer roller and the transport effect associated therewith of the ink transfer roller in connection with the printing ink still located on the surface of the ink transfer roller, over the course of time and depending on the hardness of the foreign bodies, the foreign bodies will thereby be pulverized more or less quickly, through which the doctor blades and the surface of the ink transfer roller are likewise damaged, in particular with hard foreign bodies such as grains of sand.
As a result, ink-transfer properties of the ink transfer roller thereby change in this area, with the result that at these points undesirable ink stripes occur in the printed image. If the damage to the ink transfer roller and/or to the doctor blade thereby exceed a permissible value for the achievable print quality, the ink transfer roller and/or the doctor blade must be replaced, which in particular in cases of a replacement of an ink transfer roller can result in considerable expense and in any case leads to a stoppage of the printer.
There is also during the operation of an ink transfer roller due to the continuous friction between the ink transfer roller and the downstream printing rollers and in particular due to the abrasive effect of the doctor blades on the surface of the ink transfer roller, continuous wear of the grid-like surface of the ink transfer roller, with the result that the volumes of the pockets formed on the surface of the ink transfer roller over the course of time are consistently reduced and thus the quantity of ink that can be transferred is continuously reduced. This results in a reduction of the print quality, since the ink densities of the respective print extracts are inevitably reduced.